1. Field of the Invention
The present invention relates to an optical system having a plurality of optical elements and to an imaging apparatus provided therewith, and relates to an optical system having a plurality of optical elements, such as lens components and prisms, for example, and to an imaging apparatus provided therewith, such as an imaging module for a digital camera or mobile telephone. More specifically, the present invention relates to an optical system in which an infrared-cut coating is applied to refractive surfaces of optical elements, to an optical filter, to a manufacturing method of an optical filter, and to an imaging apparatus provided therewith.
This application is based on Japanese Patent Applications Nos. 2004-235070, 2004-261301, 2005-033680, 2005-033684, and 2005-033694, the contents of which are incorporated herein by reference.
2. Description of Related Art
Recently, digital video cameras, digital cameras, and endoscopes provided with electronic-imaging optical systems including CCDs (charge coupled devices), CMOS (complementary metal oxide semiconductor) devices, etc. as imaging devices have been receiving attention.
Imaging devices formed of CCDs or CMOS devices have high sensitivity at wavelengths longer than the visible region of the spectrum (generally considered to be wavelengths from about 380 nm to about 750 nm), that is, in the so-called infrared region. Reduced resolution and image deterioration are caused by the received infrared light. Accordingly, in electronic imaging devices using imaging elements such as CCDs and CMOS devices, it is necessary to provide an infrared-cut filter or the like for removing infrared light in order to overcome this problem.
In general, the transmission characteristics of an infrared-cut filter are designed so that the transmittance of wavelengths above about 780 nm is as close as possible to zero.
Such infrared-cut filters include absorption-type filters in which the filter medium itself absorbs infrared light and reflection-type filters in which an infrared-cut coating that cuts infrared light is applied to the filter surface. In imaging apparatuses, such as digital cameras and video cameras, either of these types of filter, or both types of filter, is provided between a photographic lens and the imaging device.
Suitable methods for forming an infrared-cut coating include, for example, vacuum deposition and sputtering. In vacuum deposition, a material to be vaporized is thermally vaporized in a high vacuum and is deposited on a substrate to form a thin film. On the other hand, in sputtering, a high-energy atoms or molecules are made to collide with the material to be vaporized (target) and the atoms thrown off are deposited on a substrate to form a thin film.
An example of the transmittance-versus-wavelength characteristic of an absorption-type infrared-cut filter is shown in FIG. 36, and an example of the transmittance-versus-wavelength characteristic of a reflection-type infrared-cut filter is shown in FIG. 37. FIG. 36 shows the wavelength characteristic of an absorption-type infrared-cut filter available from Hoya Corporation (CD5000, thickness 0.45 mm). FIG. 37 is an example of the wavelength characteristic obtained when a 44-layer coating is formed on one surface of a flat filter.
When an infrared-cut filter, of either the absorption type or the reflection type, is disposed as described above, in order to ensure sufficient physical strength of the filter, the thickness must be at least 0.3 mm. Also, since it is necessary to maintain a certain spacing between the filter and lenses on either side thereof, there is a problem in that the overall length of the optical system becomes too large.
In addition, since the infrared-cut filter is provided separately from the optical system, an additional cost is required.
In order to solve these problems, Japanese Unexamined Patent Applications Nos. HEI-5-207350 and 2002-277738, for example, disclose a technique for making the system more compact by applying a multilayer film infrared-cut coating to one of the lens surfaces in a camera optical system. Also, Japanese Unexamined Patent Application No. 2004-88181 discloses a technique where an infrared-cut coating is vacuum deposited on the lens surface of a single lens. Japanese Unexamined Patent Application 2004-139035 discloses a technique for reducing the cost required for forming films by applying an infrared-cut coating to one surface of multiple glass plano-convex lenses in a batch.
Japanese Unexamined Patent Application Publication No. 2003-029027 discloses an infrared-cut filter in which a multilayer film is formed on both surfaces of a substrate, an infrared-cut filter including a plurality of substrates and having a multilayer film disposed between the substrates, and so on. By using a plurality of multilayer films in this way, it becomes easier to make the transmittance characteristic from the visible region to start of the infrared region drop. Also, infrared-cut filters having such a characteristic can be stably produced.
Japanese Unexamined Patent Application Publication No. 2004-173139 discloses an optical filter in which an infrared-cut filter is sandwiched between a first optical low-pass filter and a second optical low-pass filter.
Japanese Unexamined Patent Application Publication No. 2004-254259 discloses an infrared-cut filter in which a multilayer film is formed on both surfaces of a substrate and an infrared-cut filter including a plurality of substrates and having a multilayer film disposed between the substrates. According to this infrared-cut filter, by cutting infrared light with a plurality of multilayer films, it is possible to obtain light having wavelength characteristics close to the human visual sensitivity characteristics, and the design degree of freedom can be improved.
However, the conventional technologies described above have the following requirements and problems.
First, as well as imaging devices such as digital cameras and video cameras, recently, other products incorporating the electronic-imaging optical systems described above have also become commercially available, such as mobile telephones, mobile information terminals (PDAs), notebook computers and so on, and there is a strong demand to reduce the thickness, weight, and cost of such products. Accordingly, there are increasing demands for electronic-imaging optical systems with reduced thickness, weight, and cost that can be incorporated into such products.
Second, when applying an infrared-cut coating, there is a certain amount of error in the film thickness of each layer from the design value due to environmental variations, such as changes in temperature, humidity, and pressure. Therefore, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. HEI-5-207350, the film thickness errors are compounded when forming a multilayer film on a lens surface. Generally, in an infrared-cut coating, a large number of layers is required to obtain the desired wavelength characteristics compared to a regular antireflection (AR) coating. Therefore, the film thickness error is larger than in an AR coating, and defective products are often produced, resulting in the problem of decreased yield.
Furthermore, when using a reflection-type infrared-cut filter in which an infrared-cut coating for cutting infrared light is applied to the filter surface, a known problem is that the wavelength region that is cut differs when the light is incident perpendicularly and when it is incident at an angle. Therefore, when the infrared-cut filter is combined with a coaxial optical system that is compact in the optical-axis direction, the angle of incidence of light tends to be different between the central region and the distal regions of the infrared-cut filter, resulting in the problem that color nonuniformity occurs at the central region and the distal regions in an image acquired by the CCD or CMOS device.
In an absorption-type infrared-cut filter in which the filter material absorbs infrared light, there are almost no variations in the infrared-cutting characteristics due to the incidence angle of the light, unlike reflection-type infrared-cut filters; however, the infrared-cutting characteristics do vary depending on the filter thickness. In other words, if the filter is made thicker, more infrared light can be cut, but then it becomes difficult to reduce the thickness of the imaging optical system. Conversely, if the filter is made thinner, the imaging optical system can be reduced in thickness, but there is a drawback in that the amount of infrared light that can be cut is small.